Pitfall Trap

Oracle Text

If exactly one creature is attacking, you may pay W rather than pay Pitfall Trap's mana cost.

Destroy target attacking creature without flying.

Card Rulings

10/1/2009 You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
10/1/2009 Casting a Trap by paying its alternative cost doesn’t change its mana cost or converted mana cost. The only difference is the cost you actually pay.
10/1/2009 Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.